1. Field of the Invention
The present invention relates to an image processing method and apparatus applicable to an image forming apparatus for color printing (printing thousands of colors) such as a color printer, a color copying machine or a color facsimile apparatus.
2. Related Art
Conventionally, the full-color printers, full-color copying machines and full-color facsimile apparatuses have been put into practical use as output devices for color printing. With price reductions of these apparatuses and an improvement in quality of output images, a problem has resulted that originals which rightfully should not be copied can be easily duplicated using these machines. For this reason, a method of adding specific information such as a manufacturer's serial number of each copying machine to an output image at copying time to specify the copying machine by the serial number detected from the output image has been proposed in U.S. patent application Ser. No. 07/856,996 (filed on Mar. 24, 1992) and U.S. patent application Ser. No. 07/951,087 (filed on Sep. 25, 1992).
However, in the conventional art, an extra signal added to the image data acts as noise, degrading the quality of the output image. Especially, this degradation of image quality occurs in devices using binarization processing i.e. a pseudo half-tone processing such as Dither method or Error Diffusion method, further, in many cases, the image degradation is conspicuous.
More specifically, the conventional devices cannot adjust the density of the additional information to an inconspicuous level upon adding the information, accordingly, the additional information becomes noise in the image, degrading the quality of the output image. Further, it is inconvenient for the purpose of this arrangement that a user can recognize the additional information with the naked eye.